What if the universe was just one of many? The hypothesis is unsettling, it questions our perception of the cosmos, it invites us to rethink space-time. It completes the lesson in humility that astrophysics has taught us since the Copernican revolution 500 years ago. No, our Earth is not the center of everything, it is just one of the hundreds of billions of planets in the Milky Way, which is just one galaxy among more than 2,000 billion others in the visible universe. And now our universe itself could only be a tiny part of the multiverse! Pure science fiction? Not at all: in fact, most current cosmological models predict the existence of an abundance of these famous parallel universes so dear to the big screen…

Also read:Could there be a mirror universe on the other side of the Big Bang?

Starting even with the theory that has been a reference for more than a century: Einstein’s general relativity! This theory of gravitation holds that space-time is not fixed: it deforms and bends in the presence of matter or energy.

*“According to the theory of relativity, space could stretch to infinity or go back on itself in three dimensions, *describes Benjamin Wandelt, professor of astrophysics and member of the Planck collaboration. But if the universe is infinite, then it is mandatory: somewhere in spacetime there exists a planet similar to Earth, populated by our look-alikes, a real parallel world! This is because at infinity, anything that has a non-zero probability of occurring… necessarily occurs, and an infinity of times: there would therefore be Earths ad infinitum! It is even possible to derive the step that separates us from those closest to us: statistically speaking, 1010 29 m. A phenomenal distance, which unfortunately places it forever out of reach… Doubts? Two physical limits inherent to our space-time: the finiteness of the speed of light, which prevents us from seeing beyond 1028 m, or 46.5 billion light years; and the expansion of the universe, which distances the regions of the cosmos from each other. In this multiverse of general relativity, the parallel worlds thus follow each other in the same space-time, which unfolds in infinity…

**EXPONENTIAL EXPANSION**

Unless cosmic inflation has occurred! This second model of the multiverse is a kind of extension of general relativity. He postulates that just after the big bang, our universe would have undergone a phase of exponential expansion in which space-time would have been brutally expanded: distances would have been stretched at least 1026 times in just 1 millionth billionth of a billionth of a billionth of a second!

Constructed in 1979 by Alan Guth, a researcher at the Massachusetts Institute of Technology (MIT), in the United States, this model is now validated by the majority of cosmologists. But according to its author, this inflation was in fact eternal: it would indeed have stopped in our universe, but would continue in others, blowing bubbles of cascading universes since the dawn of time! The cosmic web of our universe would therefore not extend to infinity, it would share a boundary with a neighboring bubble universe. *“Each bubble would have basic physical properties* *various”, *describes Ranga Ram Chary, a cosmologist at the California Institute of Technology, USA. The speed of light there could be different from 300,000 km/s; the electron could be lighter there and the proton more massive; gravity more or less intense… *“Worse: If we study inflation within the framework of string theory, we can calculate that the vacuum energy in these bubbles can take 10,500 different possible configurations, *adds the researcher. *And the number of space dimensions itself could be more than three!”*

Also read:Are there universes other than ours?

**A 10-DIMENSIONAL HYPERSPACE**

Exactly, String Theory also includes a model of the multiverse… It believes that the basic constituents of matter would be tiny identical strings whose different modes of vibration would mimic the elementary particles that we know – electrons, quarks, etc. . Revolutionary consequence: our visible universe would be a membrane – or “brane” – of 3 dimensions of space, immersed in a hyperspace formed, itself, of up to … 10 dimensions, the “mass”. *“We would be trapped on one bran, but there would be many others in the bulk that would be as many universes parallel to ours,* explains Michaël Sarrazin, from the University of Bourgogne-Franche-Comté.

The physics of a hidden brane could be very close to ours, to the point that we could even find stars and planets there. And if by chance two branes approached within 1015 m along an extra dimension other than space’s three, then the particles could even jump from one universe to another, traveling in the multiverse.

MICHAEL MONNOT

The notion of the brane world is the most accomplished in terms of theoretical formalization and possible confrontation with experience –MICHAELbuckwheat,Researcher at the University of Bourgogne-Franche-Comté

So far, certainly no experimental evidence has confirmed string theory and its hidden dimensions. But Michaël Sarrazin is convinced of this: *“Among the various concepts of multiple universes, the notion of the brane world is the most scientifically accomplished in terms of theoretical formalization and possible confrontation with experience. »*

Finally, a fourth theory predicts the existence of parallel worlds: the quantum multiverse. Quantum physics has actually shown that a particle can be in two states at the same time until it is measured, causing it to switch to only one of the two states – a curiosity illustrated by the famous example of the cat by Schrödinger. Except this shift poses a problem for scientists. Why and how does the real person choose one option over the other? To solve this dilemma, astrophysicist Hugh Everett invented in 1950 *“many worlds interpretation”*, that Shrödinger’s cat was somehow dead in one universe and alive in another. During a transition, two destinies would then be fulfilled in parallel, in two disjointed realities that could never cross paths again.

EXCESSIVE NEUTRON HUNT

The Stereo collaboration attempts to detect neutrons from a source, a nuclear reactor, located on the other side of a thick wall through which they are not supposed to pass. In green, the reactor is not switched on: the detected neutrons are simply part of the environment. In red, the reactor is operating and an excess of neutrons is detected: they may therefore have succeeded in jumping into a parallel universe, avoiding the wall before returning to our reality! Alas, the error bars, in black, are still too wide to conclude on the multiverse.

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**Also read:** Does time stop in 3.5 billion years?

**Also read:** Does time stop in 3.5 billion years?

**A WORLD OF PRECAUTIONS**

Space-time would then contain all the possibilities at once. And we, poor mortals, would be sensitive to only one of them, like a radio station fixed on a certain frequency, which does not pick up the waves emitted by various stations still contained in the air. *“It’s a revolutionary idea that most people haven’t yet grasped, *notes Thibault Damour, specialist at the Institute for Higher Scientific Studies. *We must abandon the classical notion of a single spacetime or of several ‘parallel’ spacetimes: there is a single quantum world which has the character of wave superpositions.*

So of course every branch of physics has its own multiverse, its definition is variable geometry. *“Most people think of our universe as a bubble in our spacetime where the vacuum energy remains constant,” *says Ranga Ram Chary. *“The multiverse is in* *in mathematical terms a collection of interrupted spacetimes”, *for his part comments Philippe Brax, for whom everything that would be beyond the expanse of space-time consisting of all the points connected to us by light would be part of another universe*. *Michaël Sarrazin takes an even different point of view: for him the multiverse is the universe itself. *“In string theory, our visible world is definitely located on a brane, and a parallel hidden world could exist on another brane, but our universe is the totality of these branes: it is this hyperspace* *more than 4 dimensions, which we call bulk.”*

Yet all these multiverse models are compatible with each other… The reality of the physical world might well be even crazier than science fiction imagines, consisting of a layered layer of interlocking multiverses, space-time bubbles containing hidden dimensions and filled with all sorts of states for each of their particles. A meta-multiverse that one day the much sought after “theory of everything” may simply relegate to the rank of… “our universe”.

How do you prove the existence of the multiverse?

*“Theoretically, every time a neutron collides with an atomic nucleus, it can tumble into this hidden world and disappear from our brane”*, reveals Michaël Sarrazin. To test this prediction, the researcher sifted through data from the neutrino detector Stereo, at the Laue-Langevin Institute in Grenoble, looking for an excess of these wall-passing neutrons. *(see infographic) *… with no luck so far. *“Our experiments still give us an upper limit, *he reveals. *There is at most a 1 in 30 billion chance of the neutron changing to the hidden brane at the time of a collision, otherwise it would have been detected. ” *Not so easy to multi-trip.

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